In the manufacture of dental restorations, the dental practitioner prepares the tooth to be restored by grinding the subject tooth or teeth down to form one or more tooth preparations to which the prosthetic device is to be attached. An impression of the tooth preparation is taken in an elastic material and the impression is used to produce a model with dies. The model or die is then used to prepare the restoration thereon such as by casting or pressing a material onto the die.
U.S. Pat. No. 4,937,928, which is hereby incorporated by reference, is directed to a method of making a dental restoration wherein a model of a dental preparation is made by milling a refractory material under the control of a CAD/CAM system. The refractory material is a high strength material such as magnesia, which is needed to withstand the high temperatures used to process the dental materials used therein, e.g., palladium which has a melting temperature of 1552° C. Due to the high strength of the refractory material, it may be difficult to mill the material into the desired shape. Moreover, the milling tools become quickly worn and must be replaced frequently leading to high costs of production. Furthermore, magnesia is known to be unstable. It is hydroscopic and tends to absorb and react easily with moisture. Depending upon the high reactivity of magnesia, there is no guarantee that it will sinter consistently every time. The dimensions may change during sintering, resulting in a restoration which does not fit the tooth properly.
There is a need to provide a softer material for producing models and dies for the manufacture of dental restorations. It is desirable that the soft material be stable and exhibit good thermal properties. It is beneficial that the soft material be strong to withstand high temperature and pressing operations.